1. Field of the Invention
This invention relates to a magnetic toner for visualizing electrostatic latent images in an image forming process such as electrophotography and electrostatic recording. It also relates to an image forming method making use of such a magnetic toner.
2. Related Background Art
A number of methods are hitherto known for electrophotography, as disclosed in U.S. Pat. No. 2,297,691, Japanese Patent Publications No. 42-23910 (U.S. Pat. No. 3,666,363) and No. 43-24748 (U.S. Pat. No. 4,071,361) and so forth. In general, copies or prints are obtained as follows: an electrical latent image is formed on a photosensitive member by various means usually utilizing a photoconductive material, subsequently the latent image is developed with a toner into a visible image (a toner image), and the toner image is transferred to a transfer medium such as paper if necessary, and then the transferred image is fixed by heat, pressure or heat and pressure on the transfer medium.
Various developing methods to visualize the electrostatic latent images using a toner are also known. For example, there are magnetic brush development as disclosed in U.S. Pat. No. 2,874,063, cascade development as disclosed in U.S. Pat. No. 2,618,552, powder cloud development as disclosed in U.S. Pat. No. 2,221,776, fur brush development and liquid development. In these developing methods, the magnetic brush development, cascade development and liquid development, those employing a two component type developer mainly composed of a toner and a carrier, have been put to practical use. These methods are excellent in giving good images stably, but have common problems involved in the two component type developer, such as the deterioration of the carrier and change of the mixing ratio of the toner and the carrier.
To solve such problems, various developing methods employing one-component type developers comprised of a toner alone have been proposed. In particular, there are many superior methods in those employing a developer consisting of toner particles having magnetism.
U.S. Pat. No. 3,909,258 discloses a developing method employing a magnetic toner having an electric conductivity, where a conductive magnetic toner is held on a cylindrical conductive sleeve provided with a magnet inside thereof and the toner is brought into contact with electrostatic images to carry out development. In this development, in the developing zone, a conducting path is formed between the surface of the image-holding member and the surface of the sleeve via magnetic toner particles, and electric charges are led from the sleeve to the magnetic toner particles through the conducting path, and the magnetic toner particles adhere to the electrostatic image area by the coulomb force acting between the toner particles and the image area. Thus the electrostatic images are developed. This development using a conductive magnetic toner is a superior method which can avoid the problems involved in the conventional two-component type development. On the other hand, since the magnetic toner is conductive, there is a problem that it is difficult to electrostatically transfer the developed images from the image-holding member to the final transfer medium such as plain paper.
Among the developing methods employing a highly resistive magnetic toner that enables electrostatic transfer, there is a method utilizing dielectric polarization of magnetic toner particles. Such a method, however, has a problems that the development speed is substantially slow and the density of the developed images is not sufficient, thus practical use of it is difficult.
Other developing methods employing an insulating magnetic toner of high-resistivity are also known, in which magnetic toner particles are triboelectrically charged by the mutual friction between magnetic toner particles or by the friction between magnetic toner particles and the developing sleeve or the like and the toner particles thus charged come in contact with an electrostatic image-holding member to carry out development. Such methods, however, have problems in that the triboelectric charging tends to become insufficient because of the insufficient contact frequency between the magnetic toner particles and the friction member, or the charged magnetic toner particles tends to agglomerate on the sleeve because of the increasing coulomb force between the toner particles and the sleeve.
Japanese Patent Application Laid-open No. 55-18656 discloses novel jumping development that has solved the above problems, in which a magnetic toner is very thinly applied on a developing sleeve, and the toner thus applied is triboelectrically charged and brought very close to the electrostatic image to carry out development. According to this method, since the magnetic toner is very thinly applied on the developing sleeve, the contact opportunity between the developing sleeve and the magnetic toner increases enabling sufficient triboelectric charging, and also since the magnetic toner is supported by the magnetic force and the magnet and the magnetic toner are moved with respect to each other, the agglomeration of the toner particles is terminated and sufficient friction between the particles and the sleeve is achieved, whereby good images can be obtained.
However, the improved developing method employing such an insulating magnetic toner has an unstable factor due to the insulating magnetic toner used. That is, the toner contains a finely divided magnetic material mixed and dispersed in a considerable quantity and the magnetic material partly comes to the surfaces of toner particles. Hence, the properties of the magnetic material affect the fluidity and triboelectric chargeability of the magnetic toner, which consequently tend to affect various performances such as developing performance and running performance required for magnetic toners.
In the jumping development making use of a conventional magnetic toner, when the developing step (e.g., copying) is repeated for a long time, there are tendencies that the fluidity of the magnetic toner becomes lower, it is difficult to achieve normal triboelectric charging, the charging becomes non-uniform, and fogging occurs in an environment of low temperature and low humidity, thus problems occur in toner images. If the adhesion of binder resin and magnetic material that constitute magnetic toner particles is weak, the magnetic material may come off the surfaces of magnetic toner particles during the repetition of the developing step, adversely affecting the toner images, e.g., decreasing the image density.
When the magnetic material is not uniformly dispersed in the magnetic toner particles, the particles containing the magnetic material in a larger quantity and having a smaller particle diameter may accumulate on the developing sleeve, sometimes causing the decrease in image density and uneven image density called sleeve ghost.
Improvement on magnetic iron oxides to be contained in a magnetic toner has been attempted, but there is still room for further improvement.
For example, Japanese Patent Application Laid-open No. 3-67265 discloses a method to use spherical magnetic particles having a layer of a divalent metal oxide on the surface of a magnetic iron oxide particles. According to this method, in order to weaken the magnetic binding force and the magnetic cohesive force, the magnetic particles preferably have a relatively small coercive force, such as 40 to 70 oersted (3.2 to 5.6 kA/m) and also a small residual magnetization.
However, detailed studies made by the present inventors have revealed that, compared with hexahedral or octahedral particles, spherical magnetic particles when used in the magnetic toner invite increased abrasion of the photosensitive member surface because a larger amount of magnetic fine particles come to the magnetic toner particle surfaces due to the spherical particle shape.
Magnetic particles having small coercive force (Hc) and residual magnetization (.sigma.r) have a weak magnetic binding force, and hence tend to cause fog especially in an environment of low humidity.
The reason why is considered as follows. In a development means employing a magnetic toner, usually a magnet having four or more magnetic poles is provided inside the developer-carrying member (developing sleeve). When the magnetic toner jumps from the developing sleeve to the photosensitive member to form a visible image on the photosensitive member, the driving force for jumping is the quantity of triboelectricity of the magnetic toner and the controlling force against jumping is the magnetic force of the magnetic particles. When the magnetic toner particles having a large saturation magnetization come near to the magnetic poles in the developing sleeve they have a large magnetic binding force sufficient enough to control the fog phenomenon. The magnetization, however, decreases when the magnetic toner particles come to the area between the magnetic poles in the developing sleeve. Hence it is impossible to control development by the saturation magnetization. Especially in an environment of low humidity, the quantity of triboelectricity of the magnetic toner increases, and hence it becomes easy for the magnetic toner to jump to the photosensitive member, so that fog tends to occur.
Magnetic material proposed in Japanese Patent Application Laid-open No. 3-67265 is prepared by slowly adding Zn(OH).sub.2 dropwise during the oxidation reaction. Hence, the product contains a considerable amount of zinc-iron oxide inside the magnetic particles. Also, because of the large zinc content and the ample presence of zinc component inside the magnetic particles, the magnetic properties (in particular, .sigma.r and Hc) are at low values. Moreover, since the zinc component is contained in a large quantity, the developed halftone image areas tend to be yellowish when the particle diameter of magnetic toner is made as small as 8 .mu.m or less in weight average particle diameter.
Japanese Patent Applications Laid-open No. 62-279352 and No. 62-278131 disclose a magnetic toner containing a magnetic iron oxide incorporated with the silicon element. In such a magnetic iron oxide, the silicon element is intentionally positioned in the magnetic iron oxide, and there is room for further improvement in the fluidity of the magnetic toner containing the magnetic iron oxide.
In Japanese Patent Publication No. 3-9045, a silicate is added to make the shape of magnetic iron oxide spherical. In the magnetic iron oxide thereby obtained, the silicon element is distributed in a large quantity inside the magnetic iron oxide and less on the surface of the magnetic iron oxide, because of the silicate used for controlling the particle diameter, so that the improvement in fluidity of the magnetic toner tends to become insufficient.
Japanese Patent Application Laid-open No. 61-34070 discloses a process for producing triiron tetraoxide by adding a hydroxysilicate solution to triiron tetraoxide during oxidation reaction. The triiron tetraoxide particles obtained by this process have silicon element near the surface, but the silicon element is present in a layer structure near the surfaces of the triiron tetraoxide particles. Hence, there is the problem that the particle surfaces are weak to mechanical shock such as friction.
To solve the above problems, the present inventors have proposed in Japanese Patent Application Laid-open No. 5-72801 a magnetic toner containing magnetic iron oxide which contains the silicon element where 44 to 84% of the silicon element content is present on and near the surface of the magnetic material.
Such a magnetic iron oxide has brought about satisfactory improvements in the fluidity of toner and in the adhesion property to the binder resin. However, because of the local presence of silicon element on and near the surface of the magnetic iron oxide particles, such a toner tends to cause the deterioration of environmental properties, in particular, of charging property when left for a long period of time in an environment of high humidity.
Japanese Patent Application Laid-open No. 4-362954 also discloses a magnetic iron oxide containing both the silicon element and the aluminum element. There, however, is room for improvement in their environmental properties.
Japanese Patent Application Laid-open No. 5-213620 still also discloses a magnetic iron oxide containing a silicon component, where the silicon component is exposed on the surface. Like the foregoing, however, there is room for improvement in the environmental properties.
In recent years, with the digitalization of copying machines and the appearance of finer magnetic toners, higher image quality has been required for the copied images and printed images.
In copying a photographic picture containing letters, it is required that copied images of the letters are sharp and that of the picture image has a tone (image density gradation) faithful to the original. Generally, in copying a photographic picture containing letters, when the line density is increased in order to sharpen the letter images, the tone of the picture images will be damaged and also the halftone areas of the image tend to be coarse.
When the line density is increased, the amount of the magnetic toner laid on the image is so much that in the step of toner image transfer, the magnetic toner is pressed against the photosensitive member and adheres to the photosensitive member, causing what is called transfer hollow, a phenomenon caused by incomplete transfer of magnetic toner on the images, tending to provide copied images with a low image quality. On the other hand, improvement of the gradation of picture images results in a decrease in line (letter) image density, tending to decrease the sharpness.
In recent years, the tone reproduction has been improved to a certain extent by digital conversion of read image density. More improvement, however, is sought.
In addition, as the magnetic toner is made to have a smaller particle diameter, the surface area of magnetic toner per unit weight increases, which tends to bring about a broader charge distribution, thus fogging. As a result of the increase in the surface area of magnetic toner, the charging performance of the magnetic toner becomes susceptible to the influence of the environment.
When a magnetic toner has a smaller particle diameter, the dispersion states of the magnetic material and the colorant, and the magnetic properties or surface properties of the magnetic material come to affect the charging properties of magnetic toners.
Application of such a magnetic toner to a high-speed copying machine may lead to the excessive charging of the toner especially in an environment of low humidity, causing fog or the decrease of density.
It is sought to provide a magnetic toner that has overcome the various problems discussed above.